Device for connecting a station to a local area network comprising at least one ring

ABSTRACT

A device for connecting a station to a local area network comprising at least one ring includes switching arrangements associated with circuits for regenerating a signal in order to bypass a station when it is out of service and compensate the attenuation and phase jitter caused by the upstream ring segment. This device enables any number of stations to be taken out of service without seriously degrading operation of a ring. The device is usable in local area networks comprising one or more high-speed (125 Mbauds) rings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a device for connecting a station to a local areanetwork comprising at least one ring.

2. Description of the Prior Art

In a network of this kind stations are connected in series by a ring.Data is transmitted on this ring in the form of frames each comprising asender address and a recipient address. Each station retransmits eachdata frame that it receives. Conflicts between multiple stationsrequiring to send data frames are prevented by means of a special framecalled a token circulating continuously on the ring. When a stationwishes to send data it takes the token and substitutes for the tokenframe one or more data frames. It releases the token at the end ofsending these frames. The other stations merely retransmit unchanged thedata frames that they receive. The station to which a data frame isaddressed recognizes its identity in the recipient address carried bythe data frame. It stores the content of the data frame and thenretransmits the frame after adding a "received" indicator. The stationwhich sent the data frame receives it after it has travelled rightaround the ring. This station recognizes its identity in the senderaddress carried by the data frame and stops the circulation of the datacontained in the data frame.

A local area network usually comprises two rings to ensure somecontinuity of service in the event of either ring going down. Forexample, U.S. Pat. No. 5,081,452 of 14 Jan. 1992 describes a local areanetwork with one ring, a local area network with two rings and a localarea network with four rings. Each ring is supported by an electricalcable. In these three embodiments the connection device comprises aninsertion relay for each station comprising a double changeover switchoperated by a solenoid for bypassing the station when it is out ofservice in order to maintain the electrical continuity of the ring. Theinsertion relay thus enables a station or part of a station to be takenout of service without totally halting operation of the other stations.The embodiment comprising two rings enables operation of all stations tobe maintained even if one of the cables connecting two consecutivestations should break.

In the embodiment with four rings each station comprises two couplersand operation of all stations can be maintained if either coupler isdown.

Switching between the various rings is effected by electromechanicalrelays similar to that of the insertion relay for bypassing eachstation.

If any of these relays operates it causes a very short interruption tooperation of the network which is absorbed by the protocol governingexchange of data on the ring or rings.

However, consideration is being given to implementing local areanetworks at much higher bit rates. The article "The Fiber DistributedData Interface" by Floyd E. Ross, Journal of Data and ComputerCommunications, Winter 1991, pages 4-22 describes a local area networkoperating at 125 Mbauds comprising two rings supported by respectiveoptical fibers. The station connection device comprises, for eachstation and for each ring, an optical switch for bypassing therespective station when it is out of service. In the current state ofthe art the optical relays are electromechanical and have a switchingtime at the millisecond level. Switching of either of the optical relayscauses a very short term break whose duration is nevertheless very longrelative to the duration of the binary data frames transmitted on therings. This causes disturbance to operation of the network which iscompatible with some applications, such as office automationapplications, but which is not compatible with real time applications.Additionally, the electromagnetic optical relays introduce significantattenuation and phase jitter which are respectively added to theattenuation and the phase jitter generated by the ring segment betweenthe station in question and the last regenerator device on its upstreamside. It is possible to take one station out of service but it is notpossible to take a greater number of stations out of service on the samering as the accumulation of attenuation and phase jitter increase thebit error rate and may even prevent operation of the network. Theconnection device therefore enables one station to be bypassed shouldonly one station go down but does not tolerate simultaneous failure ofmore than one station or progressive installation and commissioning ofstations.

The patent application WO 83/00 238 describes a connection devicecomprising a regenerator and a switching device. It sends in thedownstream direction either a regenerated signal from the upstream sideor a signal supplied by the station connected by the connection devicein question. If the station is absent or out of service the switchingdevice sends the regenerated signal. It can bypass any number ofstations because the regenerator means compensate for the attenuationand the phase jitter caused by the ring segment on the upstream side ofthe station taken out of service.

This device still has a drawback, however: if a station is out ofservice and if the regenerator goes down transmission in the downstreamdirection is interrupted. This can happen if the local power supplyfails, for example.

An object of the invention is to remedy this drawback.

SUMMARY OF THE INVENTION

The invention consists in a device for connecting stations to a localarea network comprising one or more rings supplying signals to saiddevice, said signals being generated by said stations, said connectiondevice comprising a connection module for each station and for each ofsaid rings and said connection module comprising:

means for regenerating signals supplied by one of said rings, and

first switch means to route said regenerated signals to said stationwhen it is in service or to route said regenerated signals to a nextstation, bypassing said station when it is not in service,

each connection module further comprising second switch means forrouting the signals circulating in each ring in such a way as to bypasssaid regenerator means when said station is out of service and saidregenerator means are also out of service.

The above device can remedy failure of the regenerator means. The secondswitching means may naturally be used only for a restricted number ofstations, for the reasons already mentioned. This is not a realdrawback, however, as the second switching means are used only in theevent of failure of the regenerator means of one of the connectionmodules. This is much less probable than a station being taken out ofservice. This latter case covers the progressive installation ofstations during which many station locations may remain empty for longperiods, each location nevertheless comprising the connection meansprovided for a station. During this long period failure of theregenerator means can block the circulation of signals on one ring andit is therefore particularly advantageous to be able to remedy thisfault, even if this facility is restricted to a single connection moduleon each ring.

According to another feature of the invention the regenerator meanscomprise a narrowband filter for recovering a clock signal and filteringthe phase jitter of the signals circulating on the ring corresponding tosaid regenerator means.

This device is particularly simple to implement and can recover a clocksignal to regenerate the signal circulating on the ring when the stationis out of service and to operate the station when it is in service.

According to another feature of the invention the first switching meanscomprise means for detecting the presence of a station connected to saidmodule and for routing signals circulating on the corresponding ring ifthe station is absent.

This device enables automatic switching of the first switching means ifthe station is out of service because it is absent.

In a preferred embodiment of the invention, in the connection device fora local area network comprising at least one ring supported by anelectrical cable, the first switching means essentially comprise ahigh-speed (ECL, for example) technology multiplexer inserted into thering in series with the regenerator means and the second switching meansessentially comprise a high-speed (ECL, for example) technologymultiplexer inserted directly into the ring.

This device is faster than an electromechanical relay device and notonly enables operation to be maintained with a large number of stationsout of service but also enables a station to be bypassed in the veryrare but highly prejudicial situation in which not only a station butalso its regenerator means are out of service. In this case themultiplexer enables operation of the ring to resume after aninterruption of negligible duration and at the cost of levels ofattenuation and phase jitter which restrict to one or two the number ofstations which can be bypassed without regenerating the signal.

When the network comprises two rings each supported by a series ofelectrical conductors connecting cables grouped into a plurality ofgroups with each group plugged into a backplane carrying the connectionmodules associated with the group of stations (one module per ring andper station), in a preferred embodiment all the connection modules areidentical and each comprises:

a first ring input for connecting the module to another module by meansof an electrical cable;

a second ring input for connecting the module to another module of thesame group by means of a conductor on the backplane of said group;

means for automatically selecting the first ring input on detection ofthe presence of a cable connected to said input and for selecting thesecond ring input on detecting the absence of any cable connected to thefirst input;

a first ring output for connecting the module to another module by meansof an electrical cable; and

a second ring output for connecting the module to another module of thesame group by means of a conductor of the backplane of said group; thefirst and the second ring outputs supply two identical signals.

This preferred embodiment enables use of a single type of connectionmodule for all stations because the first ring input and the first ringoutput can be used for connection by cable, primarily to connect twodifferent groups of stations, while the second ring input and the secondring output can be used to connect two stations of the same group bymeans of a conductor on the backplane, which is a simple way to make aconnection with a given characteristic impedance.

The invention will be better understood and other details will emergefrom the following description and the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the block diagram of two connection modules of oneembodiment of a device in accordance with the invention.

FIG. 2 shows a block diagram of connections between connection modulesof a group of N stations in one embodiment of a device in accordancewith the invention.

FIG. 3 shows a block diagram of one embodiment of a connection modulewhich can be used for any station of the group except for the first andlast stations of the group.

FIG. 4 shows the block diagram of one embodiment of a connection modulewhich can be used for all the stations of the group if connectionmodules of a single type are to be used for all stations.

FIG. 5 shows the block diagram of a variant of the embodiment shown inFIG. 4 enabling the regenerator means to be bypassed.

DETAILED DESCRIPTION OF THE INVENTION

The embodiment whose block diagram is shown in FIG. 1 is suitable for anetwork with two rings. The two rings may be supported by respectiveelectrical cables if the total length of the ring does not exceed some100 meters and for greater lengths they may be supported by two opticalfibers. This embodiment can be applied to one of the networks describedin the above-mentioned article or patent application, for example.

FIG. 1 shows the connection of a single station Si but the block diagramwould be exactly the same for all other stations of the network. Thestation Si includes two send couplers CTa and CTb for coupling it to therespective rings. The coupler CTa is connected to two segments A1 andA1' of the first ring by a connection module M1a. The coupler CTb isconnected to two segments A2, A2' of a single second ring by aconnection module M1b having the same functional structure as thecoupler M1a.

The send couplers CTa and CTb both comprise two functional layers: aphysical layer protocol PHY and a medium access control layer MAC. ThePHY layer handles encoding and decoding, clock signal processing andframing of data for transmission. The MAC layer handles medium access,addressing, data integrity control, frame sending and frame receiving.The other functional layers of the station Si are not shown. Theyinclude a station management layer which handles the configuration ofthe station and of the rings and which handles all operations of thestation in the rings. These layers are implemented in conformance withthe international standard ISO/IECJTC1/SC 25.

The two connection modules M1a and M1b have the same functionalstructure and comprise a physical medium dependent (PMD) functionallayer. The PMD layer handles matching to the transmission mediumemployed, i.e. an optical fiber, for example, using components such asan optical sender and receiver appropriate to the type of medium used.

A first input of the PMD layer is connected to segment A1 which suppliesa signal arriving from the first ring. A first output of the PMD layeris connected to the segment A1' of the first ring and supplies a signalto it. A second input of the PMD layer is connected to an output of afirst switch function 1a. A second output of the PMD layer is connecteddirect to an input of the PHY layer of the coupler CTa and to an inputof the first switch function 1a. This second output supplies a signalregenerated from the incoming signal at the first input of the PMDlayer. A second input of the switch function 1a is connected to anoutput of the PHY layer of the coupler CTa.

A second switch function 2a connects the segments A1 and A1' of thefirst ring. The second switch function 2a is shown diagrammatically as aswitch because it can route signals from segment A1 direct to segmentA1' of the first bus bypassing the PMD layer of the module M1a.Likewise, a second switch function 2b in the module M1b can routesignals directly from segment A2' to segment A2 of the second ring.

The first switch function 1a is shown diagrammatically as a switchbecause when station Si is in service it can pass a signal supplied bythe PHY layer to the second input of the PMD layer and when station Siis out of service it can pass the regenerated signal supplied by thesecond output of the PMD layer to this same input of the PMD layer.

Because of its high bit rate (125 Mbauds) the signal supplied by thering is subject to attenuation and phase jitter dependent on thedistance covered from the last time the signal was regenerated. If theswitch function 1a bypasses station Si the signal transmitted by thisswitch function is subject to attenuation and phase jitter produced bythe upstream ring segment. If the signal arriving from the ring were notregenerated before being retransmitted on the ring the attenuation andthe phase jitter produced by the upstream segment would be added to theattenuation and phase jitter produced by the downstream segment and bythe first switch function 1a, respectively. The accumulation ofattenuation and phase jitter would eventually be so great that it wouldnot be possible to maintain operation of a ring with several stationsout of service. Using the connection device in accordance with theinvention any number of stations may be absent because the attenuationand phase jitter due to the upstream ring segment are neutralized byregenerating the signal in the PMD layer.

The second switch function is used in situations that are encounteredmuch less frequently, mainly in the event of failure of the PMD layer.This means that the signal must be routed without regenerating it. Thephase jitter then caused by the second switch function 2a, 2b restrictsthe number of stations that can be bypassed on the same ring using thesecond switch function to one or two and this means that the maximumdistance between stations must be restricted.

Likewise, the module M1b provides a second switch function 1b and aregeneration function similar to those just described for routing andregenerating a signal arriving on the segment A2' of the second ringbefore retransmitting it on the segment A2 when the station Si is out ofservice.

The connection modules M1a and M1b are in units physically separatedfrom the station Si so that they are present and exercise theirfunctions even if the station Si is absent.

An alternative embodiment would place the first switch function upstreamof the first regeneration function to neutralize also the attenuationand phase jitter introduced by the first switch function.

FIG. 2 shows the block diagram of the connections in a group of Nstations of a local area network which may comprise a plurality of suchgroups of stations. N=8, for example. The N stations are plugged into abackplane FP which also supports 2N connection modules M1a, M1b, M2a,M2b, . . . , MNa, MNb. In this example the maximum distance between twogroups of stations is 100 meters or less. It is therefore possible toconnect two consecutive groups of stations by means of electrical cablesCE1, CE2, CE1', CE2' comprising a screened pair for each ring. Thissimplifies the implementation as it is not necessary to use costly,bulky and insufficiently reliable optical senders and receivers. Withineach group of stations conductors of the backplane FP provideconnections between the connection modules of the various stations.

These connections operate at 125 Mbauds, for example. In this case thephase jitter affecting the signals transmitted is in the order of a fewnanoseconds to which are added a few nanoseconds distortion of the clockduty cycle. This is why it is necessary to regenerate the signaltravelling round a ring if more than two switching modules operate tobypass the associated couplers.

In this example the connection modules M1a, M1b associated with the mostupstream station S1 and the connection modules MNa, MNb associated withthe most downstream station SN are used to connect this group ofstations to two other groups of stations; the other connection modulesare used to interconnect the other stations, known as intermediatestations, of the group in question. Accordingly, the connection modulesM1a, M1b and MNa, MNb have inputs and outputs for screened cables CE1,CE2, CE1', CE2' and the other connection modules may comprise onlyinputs and outputs connected to conductors of the backplane FP.

The connection module M1a has an input for the screened cable CE1supporting one segment of the first ring. The connection module M1b hasone input for a screened cable CE2 supporting one segment of the secondring. The connection module MNa has one output for a screened cable CE1'supporting one segment of the first ring. The connection module MNb hasone output for a screened cable CE2' supporting one segment of thesecond ring. All the connection modules M1a, M1b, . . . , MNa, MNb eachhave a differential input and output respectively connected to adifferential output and input of the station associated with the modulein question by two differential lines each comprising two conductors ofthe backplane and a ground plane. Each connection module except modulesMNa, MNb has a differential output connected to a differential input ofanother module by a differential line comprising two conductors. To makethe diagram clearer the connections between the connection modules M1b,M2b, . . . , MNb and the connections between these modules and thestations S1, . . . , SN are not shown in FIG. 2. They are identical tothe connections between the modules M1a, M2a, . . . , MNa and thestations S1, . . . , SN.

Separating the connection modules from the stations means that somestations can be disconnected or not installed without interrupting theoperation of the rings. All station locations must be equipped withtheir two connection modules, however, to preserve the continuity ofboth rings.

FIG. 3 shows the block diagram of one embodiment of a connection modulesuitable only for use as an intermediate module, for example the moduleM2a. It comprises a clock signal recovery and jitter suppressor circuitDRD associated with a quartz crystal Q, a multiplexer M1 with two inputsand one output and a line sender D1. All these components areimplemented in ECL technology and all logic signals exchanged are at thelevels for this technology. The module M2a has:

a differential input EA2 connected to the connection module M1aassociated with the upstream station S1;

a differential output SA2 connected to the connection module M3aassociated with the downstream station S3;

a differential output RDATA supplying to the coupler CTa of the stationS2 the binary signal received over the ring via the input EA2 afterregenerating it (amplitude and phase);

an optical differential output RCLK supplying a recovered clock signalto the send coupler CTa of the station S2 via the backplane FP;

an optional output DS supplying a binary signal indicating detection ofa data signal on the ring to the coupler CTa of the station S2 via thebackplane FP;

a differential input TDATA connected via the backplane FP to adifferential output of the coupler CTa of the station S2 to receive inserial form a sequence of binary data to be transmitted over the ring;and

an input BP connected via the backplane FP to the coupler CTa of thestation S2 to receive a binary signal controlling the multiplexer M1.

The outputs RCLK and DS are optical as in some cases the send couplerCTa of the station can generate equivalent signals itself from datasupplied by the RDATA output of the connection module M2a. The circuitDRD exists as a commercially available integrated circuit, for examplethe Siemens V23812 or the ATT TRU-200A.

The clock signal recovery and jitter suppressor circuit DRD has oneinput which constitutes the input EA2 and three outputs whichrespectively constitute the outputs DS, RCLK and RDATA. The RDATA outputis also connected to a first input of the multiplexer M1. The secondinput of the multiplexer M1 is connected to the TDATA input. A controlinput of the multiplexer M1 is connected to the BP input. The output ofthe multiplexer M1 is connected to the SA2 output via the line senderD1.

When station S2 is present and in service it supplies to the BP input aconstant level which switches the multiplexer M1 to send the binarysignal supplied by station S2 at the TDATA input. When station S2 is outof service, and in particular if it is absent, the BP input is at adifferent level which switches the multiplexer M1 to send the binarysignal regenerated by the circuit DRD.

FIG. 4 shows the block diagram of a connection module which can beassociated with the most upstream or the most downstream station in agroup of stations or with an intermediate station of a group ofstations. In this example it is the module M1a associated with S1. Itcomprises a number of elements identical to those of the module M2apreviously described. These carry the same reference numbers. Theadditional elements are as follows:

an input EA1 via a three-way connector (not shown) for connecting ascreened pair CE1 and two further contacts K1 and K2 for connecting ashort-circuit CC integrated into the connecting cable EA1 so that it ispossible to sense the presence of the cable EA1 by detecting thisshort-circuit between the contacts K1 and K2;

a line receiver D3 which has a differential input connected to thedifferential pair of the cable EA1;

a multiplexer M2 implemented in the ECL technology, for example, andhaving three inputs and one output, the latter connected to the input ofthe circuit DRD instead of the input EA2 which is connected to a firstinput of the multiplexer M2, the second input of the multiplexer M2being connected to the output of the line receiver D3, a control inputof the multiplexer M2 being connected to the contact K1 and the contactK2 being connected to a reference potential;

an output SA1 via a connector for connecting a cable comprising ascreened pair CE2; and

a line sender D2 whose output is connected to the screened pair of thecable CE2 and whose input is connected to the output of the multiplexerM1 like the input of the line sender D1.

When the input EA1 is connected to the cable CE1 associated with theshort-circuit CC the contacts K1 and K2 are short-circuited and thecontrol input of the multiplexer M2 is therefore connected to thereference potential. This reference potential switches the multiplexerM2 to connect the output of the line receiver D3 to the input of theclock recovery and jitter suppressor circuit DRD. If there is no cableconnected to input EA1 the absence of the short-circuit CC means thatthe control input of the multiplexer M2 is not connected to thereference potential. The multiplexer M2 then sends the signal suppliedto the input EA2 by a differential line of the backplane FP from aconnection module associated with the upstream station.

In the case of the module M1a associated with station S1 there is noupstream station in the same group of stations. The module M1a isconnected to another group by a cable CE1 and the multiplexer M2 isswitched to connect the output of the line receiver D3 to the input ofthe circuit DRD. The multiplexer M2 is beneficial when it is required touse the same type of connection module for all stations S1, . . . , SN.For an intermediate station the input EA1 is not used and themultiplexer M2 automatically switches the input of the circuit DRD toreceive a signal from the input EA2.

No switching is necessary for the outputs SA1 and SA2 which supply twoidentical signals respectively supplied by the line senders D1 and D2amplifying the signal from the output of the multiplexer M1. Dependingon whether the station associated with the connection module is or isnot the most downstream station in a group, the output SA2 or the outputSA1 is used.

The function of the multiplexer M1 is as described previously withreference to FIG. 3.

FIG. 5 shows the block diagram of one embodiment of the module M1a shownin FIG. 4. This embodiment can also be used for all stations. Itcomprises an additional switch device for bypassing the clock recoveryand jitter suppressor circuit DRD when it is out of service. Aspreviously mentioned, bypassing a station without regenerating thesignal increases the phase jitter to a degree which is tolerable for onestation but which becomes intolerable if the number of stations bypassedin the same ring is increased. The additional switch device musttherefore be used only in the case of failure of the circuit DRD of oneconnection module.

The module comprises a multiplexer M3 with two inputs and two outputsimplemented in the ECL technology. The output of the multiplexer M1 goesto a first input of the multiplexer M3 instead of direct to the inputsof the line senders D1 and D2. A second input of the multiplexer D3 isconnected to the output of the multiplexer M2. The output of themultiplexer M3 is connected to the inputs of the line senders D1 and D2.A control input of the multiplexer M3 is connected to an additionaloutput of the clock signal recovery and jitter suppressor circuit DRDsupplying a constant level signal when this circuit is operatingcorrectly. When the control input of the multiplexer M3 receives thissignal it is switched to send to the line senders D1 and D2 the signalfrom the output of the multiplexer M1. If the circuit DRD is notoperating this signal is not received at the control input of themultiplexer M3 which is switched to send the signal from the output ofthe multiplexer M2 to the line senders D1 and D2. The circuit DRD isbypassed. The continuity of the ring is preserved but the signal is notregenerated.

The scope of the invention is not restricted to the embodimentsdescribed above and in particular is not limited to local area networkscomprising two rings supported by electrical cables. The man skilled inthe art will know how to apply the invention to networks comprising anynumber of rings and using another transmission medium such as opticalfiber.

There is claimed:
 1. Device for connecting stations to a local areanetwork comprising one or more rings supplying signals to said device,said signals being generated by said stations, said connection devicecomprising a connection module for each of said stations and for each ofsaid rings and said connection module comprising:means for regeneratingsignals supplied by one of said rings, and first switch means to routesaid regenerated signals to a first station, said first station beingone of said stations, when said first station is in service or to routesaid regenerated signals to a next station, bypassing said first stationwhen said first station is not in service, each said connection modulefurther comprising second switch means for routing the signalscirculating in each ring in such a way as to bypass said means forregenerating when said first station is out of service and said meansfor regenerating are also out of service.
 2. Device according to claim 1wherein said means for regenerating comprise a narrowband filter forrecovering a clock signal and filtering jitter affecting signalscirculating on one of said rings corresponding to said means forregenerating.
 3. Device according to claim 1 wherein said first switchmeans comprise means for sensing the presence of said first stationconnected to said connection module and for routing signals circulatingon one of said rings corresponding to said connection module if saidfirst station is absent.
 4. Device according to claim 1 wherein for thelocal area network comprising one or more rings supported by anelectrical cable said first switch means essentially comprise ahigh-speed technology multiplexer in said one or more rings in serieswith said means for regenerating and said second switch meansessentially comprise a high-speed technology multiplexer in said one ormore rings bypassing said first switch means and said means forregenerating when said first switch means and said means forregenerating are both out of service.
 5. Device according to claim 1 fortwo rings each supported by a series of electrical conductors connectingstations grouped into a plurality of groups each plugged into arespective backplane further supporting connection modules associatedwith each of said plurality of groups of stations wherein all saidconnection modules are identical and each comprises:a first ring inputfor connecting a first one of said connection modules to a secondconnection module by means of an electrical cable, said secondconnection module being located in a group different from said first oneof said connection modules, a second ring input for connecting saidfirst one of said connection modules to a third connection module, saidone of said connection modules and said third connection module beinglocated within one group of said plurality of groups of stations, bymeans of a backplane conductor of said one group, means for selectingautomatically said first ring input on sensing the presence of a cableconnected to said first ring input or said second ring input on sensingthe absence of any cable connected to said first ring input, a firstring output for connecting a fourth one of said connection modules to afifth connection module by means of an electrical cable, said fifthconnection module being located in a group different from said fourthone of said connection modules and a second ring output for connectingsaid fourth one of said connecting modules to a sixth connection module,said fourth one of said connection modules and said sixth connectionmodule being located within one group of said plurality of groups ofstations, by means of a backplane conductor of said one group, saidfirst and second ring outputs supplying two identical signals.